US006628921B1
(12) United States Patent
(10) Patent N0.:
Vaddiparty et al.
(54)
US 6,628,921 B1
(45) Date 0f Patent:
MULTIPLE
RETURN
SPREAD SPECTRUM
LINK
SATELLITES
CHANNEL
USER
WITH
LOADING
TERMINALS
MULTIPLE
0F
Sep. 30, 2003
370/311
370/320
455/427
455/67.11
455/12.1
6,154,450
6,222,828
6,463,279 A
B1 * 10/2002
11/2000
4/2001 Wallentin
Ohlson
Shermanetetet
al.al.
al.
6,526,260
6,567,645 B1 *
(75) Inventors: Subrahmanyam V. Vaddiparty, San
Jose, CA (US); Paul A. Monte, San
*
Jose, CA (US); Yiming Ya0, San Jose,
.t d b
C1 6
CA (US)
2/2003
5/2003 Wiedeman
Hick et al. et al.
.
y exammer
Primary Examiner—Nay Maung
Assistant Examiner—Edan Orgad
(73) Assignee: Globalstar L.P., San Jose, CA (US)
(74) Attorney, Agent, or Firm—Ohlandt, Greeley, Ruggiero
(*)
Notice:
Subject to any disclaimer, the term of this
patent is extended or adjusted under 35
& Perle, LLP
ABSTRACT
(57)
U.S.C. 154(b) by 551 days.
The satellite communication system comprises a plurality of
(21) Appl. No.: 09/687,664
(22) Filed:
Oct‘ 13’ 2000
satellites. The frequency bandwidth of the return link to each
satellite is subdivided into a plurality of channels. The
method includes steps of ?nding a total interference in each
(51) Int. Cl.7
H04B 7/ 185; H04Q 7/20
channel, calculating a predicted total interference from addi
tion of a ?rst user terminal to each channel, determining if
(52) US. Cl.
455/12.1; 455/450; 455/452;
455/427; 455/464
(58) Field of Search
455/12.1, 13.1,
455/427, 450, 509, 452, 464, 403, 422,
453, 7, 512
the ?rst channel to the ?rst user terminal. The predicted total
interference is calculated for each channel of the plurality of
(56)
predicted total interference in the ?rst channel is the mini
the predicted total interference is a minimum, and allocating
channels in the return link to each of at least tWo satellites.
The ?rst channel is allocated to the ?rst user terminal if the
References Cited
mum value.
U.S. PATENT DOCUMENTS
6,088,572 A
7/2000 Vatt et a1.
*
455/13.1
11 Claims, 9 Drawing Sheets
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US 6,628,921 B1
CREATE
INTERFERENCE [A1
DENSITY
DATABASE
T
DETERMINE
y-AZ
LOCATION
OF UT 13
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YES
ALLOCATE
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CALCULATE AVERAGE /A5
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-
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SELECT AND
ALLOCATE FDM
CHANNEL TO
UT 13' BASED ON
CURRENT INFO,
TERMINAL TYPE
AND DATA RATE
NEW
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Sheet 8 0f 9
US 6,628,921 B1
f 270
FIG.8
U.S. Patent
Sep. 30, 2003
Sheet 9 0f 9
US 6,628,921 B1
ASSIGN
CHANNEL J1
TO NEw UT
CHANNEL
wrm INTERFERENCE
<THRESHOLD
/B2
ASSIGN
CHANNEL J2
TO NEw UT
4
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CHANNEL
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ASSIGN
CHANNEL J3
TO NEW UT
=+No
DETERMINE
CHANNEL J4 WITH r58
MIN COMBlNED
INTERFERENCE
ASSIGN
CHANNEL J4
TO NEW UT
as
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FIG.9
fa?
US 6,628,921 B1
1
2
RETURN LINK CHANNEL LOADING OF
MULTIPLE SATELLITES WITH MULTIPLE
SPREAD SPECTRUM USER TERMINALS
channel of the plurality of channels in the return link for
each of the least tWo satellites. A determination of Whether
the predicted total interference is a minimum value in the
?rst channel is made With respect to all predicted total
interference values for the plurality of channels in the return
FIELD OF THE INVENTION
link. The ?rst channel is allocated to the ?rst user terminal
This invention relates in general to satellite-based com
munication systems, and speci?cally, to satellite-based
mobile telecommunication systems.
10
if the predicted total interference of the ?rst channel is the
minimum value.
In accordance With a second aspect of the present
invention, a method is disclosed for assigning a frequency
channel to a user terminal of a satellite communications
BACKGROUND OF THE INVENTION
system. The user terminal is assumed to be visible to at least
Satellite communication systems are Well knoWn in the
prior art. Examples of such systems are disclosed in US.
Pat. No. 5,303,286 and other publications that are of record
in said patent. In satellite communication systems, user
tWo satellites. The method comprises the steps of identifying
15
a location of the user terminal, determining if a ?rst fre
quency channel of a plurality of frequency channels has a
minimum total interference and, if yes, assigning the ?rst
terminals and gateWays generally communicate With each
other via one or more co-visible satellites (i.e. satellites
frequency channel to the user terminal. If not, a next step
determines if a second frequency channel has a total inter
“seen” by both the user terminals and the gateways). Some
ference beloW a predetermined threshold and, if yes, assigns
of the user terminals have broad beam antennas Which
the second frequency channel to the user terminal. If this test
illuminate much of the sky. The broad beam illumination
fails, the method then determines if a third frequency
channel has a total interference beloW the predetermined
contributes to interference With other user terminals using
the covisible satellites. Furthermore, user terminals and
gateWays of the satellite communication system may com
municate using a spread spectrum (SS) code division mul
tiple access (CDMA) technique. The nature of communica
tion using SS CDMA method is that the signal from a single
threshold for a ?rst one of the tWo satellites, and a total
25
interference above the predetermined threshold for a second
one of the tWo satellites. If yes, the method determines if the
?rst satellite is at a loWer elevation angle than the second
satellite, relative to the user terminal, and if yes, the method
assigns the third channel to the user terminal, otherWise a
user terminal is spread across the entire bandWidth of a given
communication channel. Therefore, all user terminals com
fourth frequency channel is assigned for the return link of
municating on a given communication channel may con
the user terminal. The location of the user terminal may be
tribute to interference With another user terminal communi
identi?ed When the user terminal requests service. Determi
cating on that channel. An increase in the number of user
nation of Whether the ?rst frequency channel has a minimum
terminals on a given communication channel tends to
total interference is made for the return link of the user
increase overall interference, as does an increase in any
terminal to each one of the tWo satellites. The determination
individual user terminal’s transmit poWer When it is desired 35 if the second frequency channel has a total interference
to boost the signal over the overall interference level of the
beloW the predetermined threshold is also made for the
channel.
return link of the user terminal to each satellite.
OBJECTS AND ADVANTAGES OF THE
INVENTION
BRIEF DESCRIPTION OF THE DRAWINGS
It is a ?rst object and advantage of this invention to
provide a system and method to minimiZe total interference
Within a given channel of a satellite communication system.
It is a second object and advantage of this invention to
made more apparent in the ensuing Detailed Description of
the Invention When read in conjunction With the attached
The above set forth and other features of the invention are
provide a satellite communication system having the ability
DraWings, Wherein:
45
FIG. 1 is a block diagram of a satellite communication
system that is constructed and operated in accordance With
a presently preferred embodiment of this invention;
FIG. 2 is a block diagram of the communications payload
to assign communication channels to user terminals to
achieve optimal performance of the satellite communication
system.
of one of the satellites of the satellite communication system
of FIG. 1;
FIGS. 3A and 3B respectively, are graphical representa
tions of a forWard radio-frequency (RF) link spectrum and a
return RF link spectrum used by the communication system
SUMMARY OF THE INVENTION
The foregoing and other problems are overcome and the
objects of the invention are realiZed by methods and appa
ratus in accordance With embodiments of this invention,
Wherein in accordance With a ?rst method of the present
invention, a method for maximiZing capacity of a satellite
communication system is provided. The method comprises
the steps of ?nding a total interference in each frequency
channel, calculating a predicted total interference from the
addition of a ?rst user terminal on each frequency channel,
determining if the predicted total interference in a ?rst
channel is a minimum value, and allocating the ?rst channel
of FIG. 1, shoWing the frequency division multiplexing
55
(FDM) of forWard and return link beams;
FIG. 4 is a simpli?ed block diagram shoWing a portion of
the communication system shoWn in FIG. 1;
FIG. 5 is a three dimensional matrix graphically depicting
a database of interference values per satellite per return link
FDM channel per beam of the communication system of
FIG. 1;
FIG. 6 is a How chart graphically depicting the method for
to the ?rst user terminal. The total interference is found for
allocating FDM channels to return link users of the com
each channel of a plurality of channels Which subdivide a
predetermined frequency band of a return link for at least 65 munication system of FIG. 1;
tWo satellites. The predicted total interference from the
FIGS. 7A—7C are three graphs respectively depicting the
addition of the ?rst user terminal is calculated in each
total interference density With respect to time on three
US 6,628,921 B1
3
4
different FDM channels of tWo satellites of the system of
OtherWise, the ?xed radio-telephones may incorporate mul
FIG. 1, When a neW user terminal return link is added to the
tiple antenna elements Which may be sWitched
subject channels of the tWo satellites;
(commutated).
FIG. 8 is a tWo dimensional matrix graphically depicting
the average interference density in each FDM channel per
full duplex mode and communicate via, by example, L-band
The user terminals 13 may be capable of operating in a
RF links (uplink or return link 17b) and S-band RF links
(doWnlink or forWard link 17a) through return and forWard
satellite transponders 12a and 12b respectively. The return L
band RF links 17b may operate Within a frequency range of
satellite available for use by a neW user terminal to com
municate With a gateWay of the system shoWn in FIG. 1; and
FIG. 9 is a How chart graphically depicting a sub
sequence of the method depicted in FIG. 6 for allocating
FDM channels to return link users of the communication
10
1.61 GHZ to 1.625 GHZ, a bandWidth of 16.5 MHZ. The
return links 17b are modulated With packetiZed digital voice
system of FIG. 1.
and/or data signals using a spread spectrum (SS) technique.
In the preferred embodiment, the spread spectrum commu
DETAILED DESCRIPTION OF THE
INVENTION
nications technique employs Direct Sequence (DS) spread
15
FIG. 1 illustrates a presently preferred embodiment of a
satellite communication system 10, such as for example the
(CDMA). The forWard S band RF links 17a may operate
Within a frequency range of 2.485 GHZ to 2.5 GHZ, With a
bandWidth of 16.5 MHZ. The forWard links 17a are also
GlobalstarTM system, Which is suitable for use With the
presently preferred embodiment of this invention. Although
the present invention Will be described With reference to the
embodiment shoWn in the draWings, it should be understood
that the present invention can be embodied in many alternate
forms of embodiments.
The satellite communication system 10 shoWn in FIG. 1
generally comprises a space segment 1, a user segment 2, a
modulated at a gateWay 18 With packetiZed digital voice
20
25
30
comprises a netWork of satellites 12 in LoW Earth Orbit
(LEO). The constellation of LEO satellites 12 contains an
appropriate number of satellites distributed in a suitable
GHZ to 7.075 GHZ. The satellite 12 has feeder link antennas
35
user terminals 13 and gateWays 18. Thus, a user terminal 13
may communicate from substantially any point on Earth
40
or more 10 satellites 12, possibly also using a portion of the
telephone infrastructure segment 4. In the preferred
embodiment, the satellites 12 function solely as “bent-pipe”
repeaters. As such, the satellites 12 receive communication
traf?c signals (such as speech and/or data) from user termi
frequency band and then re-transmit the converted signal.
system, if desired. Conversely, voice/data entering the gate
link 17a to the user terminal 13.
50
signals.
The user segment 2 includes a plurality of user terminals
12. Each user terminal 13 comprises a transmitting device
user terminals 13 contain the necessary baseband and RF
electronics and antennas to both transmit and receive via
band electronics to reproduce the voice/data generated at the
user terminals in digital form. The gateWay 18 interfaces the
resulting digital stream to Public SWitched Telephone Net
Work (PSTN) infrastructure segment 4. Once this voice/data
has entered the PSTN infrastructure, the voice/data is
directed to its desired destination, including back to another
Way 18 through the PSTN infrastructure is transmitted via
the forWard link 19a to the satellites 12 Which amplify,
doWn-convert from C- to S-band and re-transmit via forWard
There is no on-board signal processing of a received com
capable of operating With the satellite system 10. The user
terminals 13 include generally a plurality of different types
such as hand-held mobile radio-telephones 14, ?xed radio
telephones 16 or vehicle mounted radio-telephones 15. The
the satellite 12 and gateWay 18 are conducted. The gateWay
18 receives the return link 19b energy transmitted by all
satellites 12 Within its ?eld-of-vieW, and contains all of the
necessary RF, doWn conversion/demodulation and based
user terminal Within the referenced satellite communication
45
nals 13 or from gateWays 18, convert the signals to another
munications traffic signal. In alternate embodiments, the
satellites may be con?gured to perform some, or complete,
on-board processing of received communications traf?c
also convey satellite commands to the satellites and telem
etry information from the satellites 12. The forWard feeder
link 19a may operate in the band of 5 GHZ to 5.25 GHZ, and
the return feeder link 19b may operate in the band of 6.875
12g and 12h through Which duplex communication betWeen
number of orbital planes such that the system 10 provides
With any other point via one or more gateWays 18 and one
19 (forWard link 19a (to the satellite) and return link 19b
(from the satellite)) that operates Within a range of frequen
cies preferably in the C-band. The C-band RF links 19
bi-directionally convey the communication feeder links, and
segment 4. Satellite communication systems are described in
substantial full-earth coverage With preferably, at least tWo
satellites 12 in vieW at any given time from a particular user
location. The satellites 12 effect communication betWeen
and/or data signals using the DS-CDMA technique.
The ground segment 3 includes at least one, but generally
a plurality of the gateWays 18 that communicate With the
satellites 12 via, by example, a full duplex C band RF link
ground segment 3 and a telephone system infrastructure
US. Pat. Nos. 5,619,525, 5,758,261, 5,634,190 and 5,640,
386, Which are incorporated by reference herein in their
entirety. In the preferred embodiment, the space segment 1
ing in conjunction With Code Division Multiple Access
The ground segment 3 also comprises a Satellite Opera
tions Control Center (SOCC) 36 and a Ground Operations
Control Center (GOCC) 38. A communications path 39 is
provided for interconnecting the gateWays 18, SOCC 36 and
55
GOCC 38. This portion of the communications system 10
provides overall system control functions.
Also as shoWn in FIG. 1, the PSTN infrastructure segment
4 generally comprises existing telephone systems. For
example, the PSTN infrastructure includes Public Land
60
satellites 12 voice and/or data With the appropriate signaling
structure. The user terminals 13 are preferably provided With
omni-directional antennas 13a for bi-directional communi
Mobile NetWork (PLMN) gateWays 20, local telephone
exchanges such as regional public telephone netWorks
(RPTN) 22 or other local telephone service providers. The
PSTN infrastructure may also include domestic long dis
tance netWorks 24, international netWorks 26, private net
cation via one or more of the satellites 12. The vehicle
porate directional antennas 13b. The directional antennas on
Works 28 and other RPTNs 30.
Referring also to FIG. 2, the satellites 12 have L-band 12c
?xed radio-telephones 16 may be pointed (steered).
and S-band 12d antennas through Which full-duplex mode
mounted 15 and ?xed 16 radio-telephones may also incor
65
US 6,628,921 B1
5
6
communication is conducted between the satellites 12 and
has similar frequency structure to that of the forWard doWn
the user terminals 13. The L-band and S-band antennas are
link 17a, 19a With, for example, up to thirteen FDM
multiple beam antennas that provide coverage Within an
channels 190 centered at frequencies r1 to r13 Which are
associated terrestrial service region. For example, the
contiguously spaced Within the assigned return link 17b, 19b
bandWidths. The return link 17b, 19b, Which also incorpo
rates the DS-CDMA technique, alloWs up to, by example,
L-band 12c and S-band 12d antennas illuminate the earth
respectively With 16 beams for receiving from and 16 beams
for transmitting to the user terminals 13. Although the
128 users to transmit voice and/or data signals on each return
structure of these beams may or may not be different, the
link channel 190. In addition, each return link channel
supports signaling information from the user terminals 13 to
continuously orbiting constellation of satellites 12 provide
coverage on most of the earth’s surface 24 hours a day. As
this is an integrated World-Wide system, subscribers are
10
given the ?exibility to use their user terminals 13 anyWhere
in the World (roaming). Furthermore, in the preferred
embodiment, the LEO constellation of satellites 12 may
have more than one satellite in vieW of both (i.e. covisible)
a given user terminal 13 and gateWay 18, so that multiple
communication paths may be established betWeen them. For
the gateWay 18 including access requests, poWer change
request and registration requests. The return link 17b, 19b
generally features active closed-loop poWer control (i.e. the
user terminal’s 13 transit poWer is dynamically altered to
account for propagation effects based on received signal
15
strength at the gateWay 18). The thirteen FDM channels 190
per return link beam and sixteen beams provide for a
sixteen-fold frequency re-use for return link transmissions.
The exact number of FDM channels available for the return
link, hoWever, may vary on a regional basis depending on
example, in the satellite communication system 10, each
duplex communication betWeen a given one of the user
terminals 13 and a corresponding gateWay 18 generally
comprises a forWard link 19a, 17a (gateWay 18 to user
terminal 13) via tWo or more satellites 12 in the ?eld of vieW
of both the gateWay and user terminal, and a return link 17b,
the number of operating CDMA systems, regulatory issues,
and inter-system coordination efforts. A given user terminal
13 may or may not be assigned a different return link
channel 190 than the channel 180 assigned on the forWard
19b (user terminal 13 to gateWay 18) via the covisible
satellites 12. Thus, tWo or more satellites 12 may each
link. HoWever, When operating in the diversity reception
convey the same communication betWeen the given user 25 mode on the return link 17b, 19b (the gateWay 18 receiving
terminal 13 and the gateWay 18. Furthermore, the return and
forWard links 17b, 17a betWeen the user terminal 13 and
the user terminal’s transmission from tWo or more satellites
12), the user terminal 13 is assigned the same forWard and
return RF link channel 180, 190 for each of the satellites 12.
satellites 12 may use one or more beams of the satellites’
L-band and S-band antennas 12c, 12d illuminating the user
For both links, the gateWay 18, under allocation strategies
de?ned by the GOCC 38, or de?ned by the gateWay itself 18,
terminal. The multiple transmission paths coincident With
this mode of operation thus provides for diversity combining
is responsible for assigning the speci?c FDM channel to a
given user terminal 13. The GOCC 38 is responsible for
at the respective receivers, leading to an increased resistance
to fading and facilitating the implementation of a soft
managing all the gateWays 18.
handoff procedure. The effect of this diversity is exploited to
enhance system performance.
35
The forWard link RF spectrum (e.g. 16.5 MHZ S-band)
preferably contains thirteen different Frequency Division
Multiplexed (FDM) channels centered at frequencies f1 to
f13, Which are contiguously spaced Within the assigned
in that the latter uses coherent detection Whereas the former
uses non-coherent detection. The user terminals 13 include
multiple receivers to accept forWard RF link 19a, 17a energy
from up to three different paths using a three ?nger rake
frequency allocation.
receiver (receivers including three distinct RF/IF/
Demodulation paths). In the return link 17b, 19b, the gate
It should be noted that the forWard link RF spectrum may
contain any number of channels and each channel could
Way 18 may have up to a seven ?nger rake receiver, thereby
have a different bandWidth (e.g., 1.25 MHZ, 3.75 MHZ, etc.).
FIG. 3A shoWs a graphical representation of the FDM
channels 180 subdividing the beams of the forWard doWn
link 17a (satellite 12 to user terminal 13). The frequency
structure of the forWard uplink 19a from the gateWay 18 to
The return link 17b, 19b in the satellite communication
system 10 may be different than its forWard link 19a, 17a,
45
non-coherently combining return RF link energy through up
to seven different paths. As noted previously, these paths
may convey energy betWeen a single gateWay 18 through
several satellites 12 and/or several beams through one
satellite 12.
the satellites 12 (not shoWn) is substantially similar to that
The near omni-directional antennas of hand-held user
of the forWard doWnlink 17a. The FDM channels 180 are,
terminals 14 and vehicle mounted user terminals 15 illumi
for example, 1.23 MHZ Wide in frequency. Each of these
thirteen FDM channels contain multiple voice and/or data
nate the sky almost uniformly. This broad beam illumination
in the return uplink 17b impinges on the covisible satellites
and some overhead functions such as a pilot, paging and
12 and contributes to interference on the return link FDM
channels 190. The level of interference on the return link
synchroniZation signals. The thirteen FDM channels per
forWard link beam, and sixteen beam structure of the for
Ward link antenna 12d, provides for a sixteen-fold frequency
re-use for forWard link transmissions. The preferred
DS-CDMA communication technique Which is used When
55
across the entire bandWidth of a given FDM channel.
Therefore, all users Within the FDM channel may represent
transmitting these signals employs up to, for example, 128
different Walsh spreading codes Within each FDM channel.
This alloWs a variable number of users to simultaneously
occupy the same FDM channel. The gateWay 18 transmits
the appropriate amount of poWer through the satellites 12,
and by means of link quality measurements at the user
terminals 13, the transmit poWer is dynamically adjusted to
achieve optimal link-by-link performance.
The return link 17b, 19b frequency plan is graphically
depicted in FIG. 3B. The return link 17b, 19b RF spectrum
FDM channels directly determines the capacity of the return
link FDM channels. Generally, CDMA modulation tech
niques spread the signal from an individual user terminal 13
65
interference to the signal of interest, unless the other signals
are otherWise orthogonal (in code space) to the signal of
interest. In the forWard link 19a, 17a, all signals Within an
FDM channel 180 are assigned orthogonal Walsh codes by
the gateWay 18. In the return link 17b, 19b the overall
interference, and hence the capacity of one FDM channel
190, is generally dependent on the signal-to-noise-ratio
(SNR), or its equivalent in the digital domain, energy-per-bit
to noise-density
US 6,628,921 B1
8
7
resulting system capacity is thus not maXimiZed, Which in
turn raises the transmit poWer demands on the user terminals
(i.e. the system performance is non-optimal).
The effects of clustering by user terminals on system
performance are mitigated in the present invention by selec
ratio. The terms Eb, N0, and I0 respectively represent; the
tively assigning return link channels to the user terminals.
In the preferred embodiment, the GOCC 38 has a master
received power per data rate (i.e. energy per bit), the thermal
noise density and the total interference noise density (in 1 HZ
of the FDM channel bandWidth). The interference density
(I0) is a function of the number of user terminals 13 using the
FDM channel (i.e. system capacity) and their corresponding
10
transmitted RF poWer. The term (IO/N0) represents the
additional degradation in a given FDM channel of a given
satellite 12 from the ideal no interference (I0=0) case and
provides a convenient metric in evaluating the return link
performance.
15
As the number of users in an FDM channel increases, by
controller 380 Which allocates a return link FDM channel to
each user terminal in accordance With the method described
beloW. In alternate embodiments, one or more of the gate
Ways may have a controller to allocate the return link FDM
channel to the user terminals. Preferably, the master con
troller 380 is aWare of the type and location of each of the
user terminals 13 in communication or initiating communi
cation With the gateWay. For eXample, the gateWay 18 may
have a capability of detecting and tracking the location of
each user terminal With Which the gateWay is communicat
example, to increase system capacity, then the overall inter
ing. This may be accomplished by an appropriate locating
ference increases. In order to achieve the appropriate energy
per bit to noise density rate
algorithm programmed into the gateWay Which uses the
signals relayed by multiple satellites to locate the user
terminal on the earth’s surface. OtherWise, the user terminal
may include a position determining device, the location data
from Which may be transmitted by the user terminal on one
of the return link overhead channels. The user terminal type
on the FDM channel, it may be desirable to increase the
transmitted poWer from the user terminal 13. The higher
transmit poWer from the user terminal in turn further
25
the gateWay 18 during registration (and from the gateWay 18
to the master controller 380). Preferably, the master con
troller 380 may also be aWare of the position, at any given
time, of all the satellites 12 in the constellation of satellites
increases the interference to other UTs on the same FDM
channel.
The gateWay 18, either directly or otherWise under control
of the GOCC 38, allocates the resources of the satellite
communication system 10 (i.e. satellites 12 and FDM chan
nels 180, 190) to the forWard 19a, 17a and return links 17b,
19b to achieve optimal operation of the system. Examples of
systems and methods for allocating satellite communication
(i.e. vehicle mounted or hand-held) may be included in the
information signals transmitted by the user terminal 13 to
of the communication system 10 as Well as the number of
35
system resources to forWard link users are described in US.
FDM channels available in the region of the earth illumi
nated by each satellite’s return link antenna 12c‘, 120“.
Satellite position data may be established from telemetry
data transmitted by the satellites to the gateWay. The master
controller 380 may otherWise be programmed With addi
Pat. Nos. 5,592,481 and 5,812,538 incorporated by reference
herein in their entirety. In the present invention, the gate
Ways 18, either directly, or under control of the GOCC 38,
tional system architecture parameters as Well as other ancil
assign the return link users to speci?c FDM channels 190
such that the total interference is minimiZed Within the
embodiment, the gateWay 18 may be aWare of the position
of all the satellites 12 in the constellation of satellites of the
communication system 10 at any given time. The gateWay
lary information to facilitate selection of the FDM channels
as Will be described in further detail beloW. In an alternate
assigned FDM channel and performance of the system is
optimiZed.
may also be aWare of the number of FDM channels available
Referring noW to FIG. 4, there is shoWn a simpli?ed block
diagram of a portion of the satellite communication system
10. The present invention Will be described With speci?c
reference to this portion of the satellite communication
system 10 shoWn in FIG. 4, though the present invention
applies equally to the Whole system. In FIG. 4, a number of
45
in the region of the earth illuminated by each satellite’s
return link antenna 12c‘, 12c“. In addition, the gateWay 18
may further be programmed With additional system archi
tecture parameters as Well as other ancillary information to
facilitate selection of the FDM channels as Will be described
user terminals 13, 13‘ are shoWn transmitting signals to one
in further detail beloW.
Referring noW to FIG. 6, there is shoWn a How chart
gateWay 18 through tWo satellites 12‘, 12“ visible to both the
Which graphically illustrates the method for allocating FDM
user terminals and the gateWay. Due to orbital geometry,
each of the tWo satellites 12‘, 12“ is at a different elevation
relative to a given user terminal 13, 13‘. The user terminals
channels to return link users. An overvieW of the method is
13, 13‘ are distributed on the earth’s surface so that each user 55
terminal is illuminated by one or more beams of the L-band
antenna 12c‘, 12c“ on each of the tWo satellites 12‘, 12“. A
return uplink 17b is established betWeen each user terminal
13 and each satellite 12‘, 12“. Each satellite receives the
signals from each transmitting user terminal 13 via the
return uplink. Each satellite then repeats the return link
signals and transmits them to the gateWay 18. Generally, the
user terminals 13 are not uniformly distributed, but rather,
tend to cluster in geographic regions on the earth’s surface.
This clustering may lead to some beams illuminating the
earth from the satellite L-band (return link) antenna 12c‘,
12c“ being heavily utiliZed While others remain falloW. The
substantially as folloWs. First, in step A1 of FIG. 6, a
database of the interference density to thermal noise ratio
(IO/N0) for every return link FDM channel 190 into each
satellite 12 is initially created at some initial time to. After
this database is created, in step A2 the location and type of
a given user terminal 13‘ is determined When the user
terminal 13‘ makes a request for service to the gateWay 18.
The request for service may include a requested data rate.
During a session the data rate request can be made Which
increases or decreases the current data rate. This may occur
multiple times during a connection.
With the location and type of the neW user terminal 13‘
65
(i.e. the user terminal requesting service) identi?ed, then in
step A3 a determination is made as to the geographical
distribution of other user terminals 13 communicating With
US 6,628,921 B1
9
10
the gateway 18 via the same satellites 12 as the neW user
eXample, adjustments to the values in the data base 200
terminal 13‘. If it is determined that the user terminals 13, 13‘
could be made every minute either in near real time at the
are substantially evenly distributed, then in step A4 of FIG.
gateWay 18 and/or in a predictable mode at the GOCC 38.
6, an FDM channel is allocated to the neW user terminal 13‘
Initially, the (IO/N0) values in the data base 200 may be
established by either folloWing the methods described beloW
such that all the FDM channels have a substantially uniform
user distribution. HoWever, if it is determined that the user
from the start of service, or otherWise derived analytically
based on a priori knoWledge of the locations of the user
terminals 13 and satellites 12 at a given time, based on
terminals 13, 13‘ are not uniformly distributed
geographically, then in Step A5, the average effective addi
tional interference (AIO/NO) from the neW user terminal 13‘
is calculated for each return link FDM channel into the
satellites used by the neW user terminal 13‘. From the
suitable return link analyses techniques.
10
additional interference, the appropriate FDM channel is
Whenever a request for service is made by the user terminal
to the gateWay 18. The request for service may be made in
selected and allocated in Step A6 to minimiZe the total
interference on the FDM channels of the satellites 12 as Will
be described in greater detail beloW. After FDM allocation
Referring noW to FIGS. 3B, 4 and 6, the location of a
given user terminal 13‘ is determined in Step A2 of FIG. 6,
response to a need to establish a communication link
15
in step A6, the user terminal 13‘ may request an increase or
decrease in the current data rate as mentioned above, and as
shoWn in step A7 of FIG. 6. In the event the user terminal
13‘ makes such a request, the average effective additional
betWeen the user terminal 13‘ and gateWay 18, or may be
generated so as to handoff an already established link from
one satellite to another. Generally, the request for service is
made by the user terminal 13‘ at some time t after the initial
epoch to. The location of the user terminal 13‘ requesting
service is used in conjunction With information otherWise
interference (AIO/NO) from the user terminal 13‘ is again
calculated in step A5 for each return link FDM channel into
the satellites used by the user terminal 13‘. From the addi
tional interference, an appropriate FDM channel is again
selected and allocated in Step A6 to minimiZe the total
stored in the master controller processor or the gateWay 18
to determine, at time t, Which satellites 12‘, 12“ are visible
to that user terminal 13‘, the corresponding beams of the
change request multiple times during a connection.
terminal, the type of user terminal 13‘ requesting service (i.e.
For each neW terminal requesting service, steps A2—A6 of
the above described procedure are repeated as necessary.
The database created in Step A1 of FIG. 6, is generated,
using an appropriate processor in the master controller 380
hand-held or vehicle mounted radio-telephone) is also estab
lished in step A2 of FIG. 6. From the type of user terminal,
the master controller 380 of the GOCC 38, or the gateWay
satellites illuminating the user terminal 13‘ as Well as the
interference on the FDM channels of the satellites 12. As 25 number of FDM channels 190 otherWise available to the
user terminal 13‘. In addition to the location of the user
mentioned above, the user terminal 13‘ may make a data rate
18, may then determine the link closure requirements (eg
energy-per-bit to noise-density ratio, antenna
of the GOCC 38. In an alternate embodiment, the database
created in Step A1 of FIG. 6 may be generated by a
controller in the gateWay 18. The database includes values
for the ratio of interference density to thermal noise density
(IO/N0) for each FDM channel 190 Within each return link
beam of each satellite 12 in the constellation of satellites of
the communication system 10. Aschematic representation of
this database 200 of (IO/N0) values is shoWn in FIG. 5 as a
three dimensional matrix With the beams and FDM channels
characteristics) Which are different for different types of user
terminals. The link closure requirements can be used to
35 either miX or segregate user terminals 13 Within an FDM
channel.
If in Step A2 it is determined that all user terminals 13, 13‘
visible to the given satellites 12‘, 12“ are substantially
uniformly located on the ground, then the return link FDM
channel 190 is allocated to the neW user terminal 13‘ in step
arranged respectively in roWs and columns arrayed by
satellite. This database 200 generally represents the net
A4 to distribute the transmitting user terminals substantially
uniformly on all FDM channels 190. In this case, the
interference status on all the FDM channels 190 of the return
uniform assignment approach may be appropriate to mini
links 17b into each of the satellites 12 of the satellite
constellation of the communication system 10 (see also FIG.
45
miZe the total interference density to thermal noise ratio
(IO/N0) Within any FDM channel into the satellites 12‘, 12“.
If the traffic through the gateWay 18 serving the given
geographic region does not Warrant the full complement of
1). The data base is created at some initial time or epoch
(to=0) during operation of the satellite communication sys
tem 10. This initial time may coincide With the start of
available FDM channels the number of available FDM
service of the communication system 10. The thermal noise
density N0 is a predetermined value Which is a function of
the satellite’s 12 communication payload and is otherWise
registered in the master controller processor of the GOCC 38
or a controller in the gateWay 18. The thermal density N0
channels may be reduced accordingly in that region. Reduc
ing the number of available FDM channels reduces the cost
of the gateWay 18 due to reduced hardWare, softWare and
may be identi?ed, for eXample, a priori from ground testing
uniformly distributed on the ground, the neXt step (i.e. step
A5 of FIG. 6) is to calculate the average (over time) effective
of each satellite’s communication payload. The interference
density I0 on each FDM channel of each beam may be
maintenance requirements.
If the user terminals 1 are determined, hoWever, not to be
55
additive interference ratio (AIO/NO) the neW user terminal 13‘
Will add if it is assigned to any one of the available return
referenced, by example, at the return uplink Low-Noise
Ampli?er (LNA) (not shoWn) Which is part of the L-band
link FDM channels 190. The normaliZation factor (thermal
density N0) is arrived at as previously described. The
antenna 12d of each satellite 12. OtherWise, the interference
density I0 may be referenced anyWhere Within each satel
additive interference density (AIO) of the neW user terminal
13‘ is calculated for all return link FDM channels 190 (in this
case there are thirteen FDM channels though this number
lite’s 12 communication electronics chain. The data base
200 of (IO/N0) values is periodically updated. Each of the
interference density to thermal noise (I O/NO) values in the
may vary) of all covisible satellites 12‘, 12“ (in this case
data base 200 is dynamically adjusted over time relative to
the initial epoch (to=0). The adjustments to the (IO/N0)
values may be performed at some pre-de?ned time incre
ments by either the gateWay 18 or the GOCC 38. For
there are tWo covisible satellites though this number may
65
also vary) through Which the return link 17b, 19b to the
gateWay 18 may be established. The additive interference
density is preferably calculated by the master controller 380
US 6,628,921 B1
11
12
of the GOCC 38. The additive interference (AIO) is generally
de?ned by the ratio (Pr/ri) Where Pr represents the power of
The transmit poWer of stations for other RF services
the neW user terminal’s 13‘ transmission received at each
(factor
operating proximate to the location of the neW
user terminal 13‘, such that they may potentially interfere
covisible satellite L-band antenna 12d and ri is the band
With the terminal’s transmissions, is generally predicted by
Width (e.g. 1.23 MHZ) for each FDM channel (i=1—13 in this
the master controller 380 of the GOCC 38 using one or more
case). The transmission received poWer P, is in turn gener
ally related to the transmit poWer Pt of the user terminal 13‘
requesting service. The transmit poWer P, demanded of the
user terminal 13‘ so that it may be assigned to any of the
of the folloWing methods. For eXample, the master controller
380 may be programmed With information identifying
potentially interfering RF services (eg those RF services
eXpected to be using radio frequencies proXimate the L-band
available FDM channels 190 may be determined using
conventional return link closure analyses techniques.
bandWidth used by the return link 17b of the communication
system 10) around the World. From this pool, the master
For example, the poWer P, of the user terminal 13‘ to
transmit on any FDM channel 190 is generally a function of
factors such as: a) the range betWeen transmitter and
controller 380 identi?es those services suf?ciently proXi
receiver and corresponding space loss; b) the gain and losses
mate geographically to the location of the neW user terminal
15
of the user terminal’s antenna 13a and satellite’s L-band
antenna 12c‘, 12c“ (in particular the gain of the L-band
antenna beam Where the neW user terminal 13‘ is currently
located); c) L-band antenna 12c‘, 12c“ beam ef?ciency; d)
the average data rate of the user terminal 13‘; e) voice
13‘, identi?ed in Step A2, to cause interference. The master
controller 380 of the GOCC 38 then establishes the number
of transmitting stations and characteristics associated With
these services. OtherWise, the master controller 380 may use
a predictive factor for these systems that includes some
assumptions With respect to the number and characteristics
of the RF services potentially interfering With the user
terminal 13 transmissions. (eg A reasonable assumption
activity effects; f) the overall interference on the FDM
channels 190; g) the transmit poWer of stations transmitting
to other RF services in the geographical location of the neW
may be that the number and characteristics of other system’s
stations are equal to those of the satellite communication
user terminal 13‘ and h) the eXpected duration of transmis
system 10. A scaling factor may also be applied based on the
sion of the neW user terminal 13‘. The master controller 380 25 assumption that the other system’s transmit poWer may be
of the GOCC 38 is suitably programmed to quantify the
above listed factors from ephemeral data received from the
scaled as the square of the ratio of the altitude (or average
of the slant ranges) of the other system’s satellites to the
altitude (or average of the slant ranges) of the satellites 12
user terminals 13, the satellites 12 and gateWays 18, or
otherWise from data registered in the master controller
processor.
It should be noted that in an alternate embodiment the
of the communication system 10.)
It should be noted that, in an alternate embodiment, the
transmit
may be predicted
poWer of by
stations
the gateWay
for other18.
RF services (factor
gateWay 18 may calculate the additive interference density
and further may be suitably programmed to quantify the
The master controller 380 of the GOCC 38 employs the
above listed factors from ephemeral data received from the
above listed factors in the return link closure analysis to ?nd
user terminals 13, the satellites 12, or otherWise from data
35
registered in the gateWay 18 itself.
In this case, for eXample, the range (factor (a)) betWeen
the transmit poWer demand Pt on the neW user terminal 13‘
so that it may transmit on any FDM channel 190 of each
the transmitter and receiver is calculated from the location of
the user terminal 13‘, identi?ed in Step A2, and that of each
relaying satellite 12‘, 12“. The received poWer P, at the LNA
of the satellite’s L-band antenna 12c‘, 12c“ is then
calculated, also for each FDM channel, based on the user
relaying satellite 12‘, 12“ registered previously in the gate
terminal’s transmit poWer Pt adjusted by the path gain (i.e.
Way 18 and/or the master controller 380. The L-band
space loss and antenna characteristics; previously identi?ed
antenna beam ef?ciency (factor (c)), (i.e. the roll-off char
factors (a) and
acteristics of neighboring beams from Which unintended
energy is impinging into the FDM channel Within the beam
illuminating the neW user terminal 13‘) is otherWise estab
value into each FDM channel 190 at each satellite 12‘, 12“
may then be determined from the ratio (P,/ri) of the received
poWer at the LNA of the satellite’s L-band antenna 12c‘,
12c“ to the FDM channel bandWidth. The additive interfer
ence (AIO) represents the increase in interference into each of
the available FDM channels 190 Within each of the covisible
45
lished through prior testing of the L-band antenna 12c‘, 12c“
and then registered in the master controller 380. The average
transmission data rate (factor
and the voice activity
effects (factor (e)) of the terminal are quanti?ed from
Finally, the additive interference (AIO)
satellites 12 from addition of the neW user terminal 13‘. The
additive interference is normaliZed by the thermal density
value No to ?nd the additive interference density (Ale/N0).
By evaluating (AIO/NO) at each satellite’s L-band antenna
12d and using this parameter as the metric of comparison
predictive models (Which state What an average user termi
nal may transmit for different percentages of time at different
data rates and identify a margin on the average data rate to
account for instantaneous data rates different than the aver
attempt to account for the time period that the neW user
across all the FDM channels 190, the effect of different path
losses (i.e. space loss and antenna characteristics) and user
terminal transmit poWer Pt are substantially accounted for.
The return link closure analysis, as described above, Will
yield a series of values of the additive interference density
(AIO/NO) Which may be plotted With respect to time per FDM
channel 190 to account for effects arising from movement of
the relay satellites 12‘, 12“ With respect to the neW user
terminal 13‘. The effects of the satellite’s movement (i.e.
terminal 13‘ Will be transmitting at the average data rate (in
orbital rotation) relative to the neW user terminal 13‘ on the
age value). The overall interference (factor
on the FDM
channels (an indication of the number of user terminals 13
already active Within each of the FDM channels) is identi
?ed from the database matriX 200 (see FIG. 5) created in
Step A1 and updated as described further beloW. The
eXpected duration of transmission (factor
is a value also
generated preferably by the master controller 380 of the
GOCC 38 based on accepted predictive methods Which
55
this case a period of tWo minutes may be selected though,
other time periods may be chosen as desired). Alternatively,
the eXpected duration of transmission (factor
may be
generated by the gateWay 18.
65
interference density of the FDM channels 190 is shoWn in
FIGS. 7A—7C. FIGS. 7A—7C are three graphs of the total
interference density (IO+AIO)/NO plotted over time for three
representative return link FDM channels (i.e. the ?rst 190A,
US 6,628,921 B1
13
14
the second 190B and the thirteenth 190M FDM channels,
see also FIG. 3B). Each graph shows a set of curves (121,
122 in FIG. 7A; 123, 124 in FIG. 7B and 125, 126 in FIG.
7C), each curve in the set corresponding to the particular
FDM channel received into one of the relay satellites 12‘,
12“. In this case, there are tWo relay satellites 12‘, 12“ and
referred to hereafter as p(j,k) Where: p=(IO+AIO)/NO; j cor
responds to a speci?c return link FDM channel (e.g. j=1 for
the ?rst FDM channel 190A, j=2 for the second FDM
channel 190B and so on to j=13 for the thirteenth FDM
channel 190M of the return link) and k corresponds to a
speci?c satellite (e.g. k=1 for satellite 12“, k=2 for satellite
hence tWo curves per set. The total interference density
(IO+AIO)/NO represents the cumulative interference into each
FDM channel 190 from the user terminals 13 already active,
at time t, Within each of channel (this base interference
density (IO/N0) value is given by database 200) and the
10
additive interference density (Ale/N0) of the neW user ter
minal if added to each channel. Each curve in the graphs of
FIGS. 7A—7C shoWs the change to the total interference
12‘). Therefore, the parameter {u} may be de?ned as a tWo
dimensional [13x2] matriX 270 as shoWn in FIG. 8, because
there are, by eXample, thirteen available FDM channels
190A—190M (j=1—13) and tWo visible satellites 12“, 12‘
(k=1,2) to the user terminal 13‘.
The master controller 380 of the GOCC 38, or in the
alternative, the gateWay 18, in step A6 of FIG. 6, selects and
density (IO+AIO)/NO per channel per satellite due to relative
motion of the satellite With respect to the neW user terminal 15
13‘ over the eXpected duration of the call (for example, tWo
minutes). Here, the graphs of FIGS. 7A—7C portray the case
assigns the appropriate FDM channel 190 to the user ter
minal 13‘ requesting service in accordance With the proce
dure described beloW With reference to the How chart in FIG.
9. Thus, the procedure depicted in the How chart of FIG. 9
is a sub-sequence included in step A6 of FIG. 6. In step B1
Where one satellite 12“ is retreating and the other satellite 12‘
is approaching the neW user terminal 13‘ along their orbital
paths. Thus, referring to FIG. 7A, if the user terminal 13‘
Were assigned to the ?rst FDM channel 190A (also see FIG.
of the How chart in FIG. 9, a determination is made as to
Whether there is a common FDM channel j1 to each satellite
12“, 12‘ (eg channel 1,1) and 1,2)) such that the predicted
3B), the total interference to thermal noise density in the ?rst
average total interference
channel 190A of satellite 12“ may be mapped as curve 122,
and in the ?rst channel 190A of satellite 12‘ as curve 121.
to each satellite is the minimum average total interference
Curve 122 is decreasing (i.e. decreasing interference) over
25
the call duration because satellite 12‘ is moving closer to the
user terminal 13‘ (presenting a higher elevation angle rela
tive to the user terminal). Curve 121 is increasing (i.e.
increasing interference) because satellite 12‘ is moving far
ther aWay (presenting a loWer elevation angle relative to the
user terminal). The interference curves in FIG. 7B (curves
123, 124) and in FIG. 7C (curves 125, 126) are similar in
curvature (i.e. rate of change) to those in FIG. 7A, though
the magnitudes may be different. Similar sets of curves
Would be generated by the master controller 380 of the
GOCC 38 for each return link FDM channel 190 (in this
35
1,1) and
1,2) of that channel
p(j,k) from the channels 190A—190M (j=1—13) in the return
link 17b to the corresponding satellite 12“, 12‘ (i.e. p¢(j1,1)
§p(1—13,1) and p¢(j1,2)§p(1—13,2)). If there is a channel j1
common to both satellites (i.e. 1,1) and 1,2)) having the
minimum interference density p in comparison to the other
channels to the corresponding satellite 12“, 12‘, then in step
B2, the neW user terminal 13‘ is assigned the channel jl. If,
hoWever, the channel having the loWest interference density
in each satellite is not the same channel (e.g. p¢(1,1) is the
loWest interference density in satellite 12“ but /4(5,2) is the
loWest interference density in satellite 12‘) then in step B3,
a determination is made as to Which FDM channel j2 to both
thirteen return link channels per satellite times tWo visible
satellites has an average interference density p(j2,1) and
p(j2,2) less than a predetermined threshold value. This
threshold value may be determined from system simulations
or based on operational (trend analyses) data and modi?ed
as appropriate. If such an FDM channel j2 is found in step
B3, then in step B4, the neW user terminal 13‘ is assigned to
the FDM channel j2.
satellites). The result is a predicted average total interference
on each return link channel 190 into each relay satellite 12“,
lites having an interference density p(j2,1) and p(j2,2) less
case, thirteen sets of curves Would be generated, one for each
of the thirteen FDM channels). The master controller 380 of
the GOCC 38 then averages (With respect to time) each of
these curves leading to, in this case, tWenty siX averaged
predicted total interference values (one for each of the
OtherWise, if there is no channel common to both satel
12‘ from transmission by the neW user terminal 13‘. The rate 45 than the predetermined threshold value, then in step B5 a
determination is made Whether the average interference
of change (i.e. curvature) of the interference curves 121“126
in FIGS. 7A—7C is shoWn only for eXample purposes and
may be different in actuality. For example, if the satellite
communication system 10 of the preferred embodiment has
density of the channel j3 in one satellite, for example, p(j3,2)
in satellite 12‘ (k=2), is less than the threshold (the average
interference density p¢(j3,1) of the comparable channel in the
other satellite 12“ (k=1) being greater than the threshold). If
active poWer control, as it eXists in a satellite communication
system such as the GlobalstarTM system, the rate of change
in the curves may be signi?cantly loWer (and Will actually be
?at in the case of ideal poWer control). In addition, the set
Yes, then in step B6, a further determination is made Whether
the elevation angle betWeen the user terminal 13‘ and
of curves for each channel need not have one curve With a
higher interference density p(j3,1), is higher than the eleva
tion angle to satellite 12‘ (k=2) having the channel j3 With the
loWer interference density p(j3,2). If Yes, then in step B7 and
positive (increasing) rate of change and the other With a
negative (decreasing) rate of change. Both curves may have
satellite 12“ (k=1), corresponding to the channel j3 With the
55
the user terminal 13‘ is assigned to that FDM channel j3. This
negative a rate of change, as in the case Where both satellites
are approaching the user terminal 13‘. Alternatively, in the
case Where both satellites are retreating from the user
terminal, both curves may have a positive rate of change.
results in the satellite 12“, at the higher elevation angle
having (see FIG. 4) to suffer greater interference. This is
acceptable because visible satellite 12‘ at the loWer elevation
angle presents a Worse overall path to the signals and
While the return link closure analysis is described above
as being performed by the master controller 380 of the
GOCC 38, it should be noted that, in an alternate
embodiment, that the return link closure analysis may be
performed by the gateWay 18.
For notational simplicity, the average eXpected total inter
ference density per return link channel per satellite Will be
therefore needs to have a loWer overall interference.
65
If the ansWer in step B5 is No, (i.e. the channels to both
satellites 12“, 12‘ have an average interference density p(j,k)
greater than the threshold value) steps B6 and B7 are
skipped and step B8 is performed. As shoWn in FIG. 9, step
B8 is also performed if the ansWer in step B6 is No (i.e. the
US 6,628,921 B1
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16
satellite 12“ corresponding to the channel j3 having an
interference density below the threshold value has a higher
elevation than the satellite 12‘ With the channel j3 having an
identi?es the total interference density per channel per
satellite as previously mentioned. As also mentioned above,
the information from the updated database matrix 200 may
be obtained from the master controller 380 of the GOCC 38,
interference density above the threshold value). In step B8,
a determination is made as to Which channel j4 has the
or in an alternative embodiment may be obtained by cir
minimum combined average interference density
4,1)+/1
(j 4, 2)) across both satellites 12“, 12‘ (k=1—2) from the
combined interference density (,u(1—13,1)+p(1—13,2)) across
cuitry in the gateWay 18.
The type of the user terminals already communicating on
the satellite is registered by the master controller 380 (i.e.
from the data obtained in step A2 of the channel allocation
both satellites for all available channels 190A—190M
(j=1—13). Then, in step B9 of FIG. 9, the user terminal 13‘
is assigned to this FDM channel j4. This is essentially a
10
?xed radio-telephone 16‘ With a steerable directional
antenna requests service from the gateWay 18, the master
controller 380 of the GOCC 38 locates the position of the
terminal and identi?es the terminal as being a ?xed radio
fall-back position Which Will assure that, on an average, the
visible satellites 12“, 12‘ Will experience loWer interference.
After the neW user terminal 13‘ is assigned a communi
cation channel, the interference database represented by
matrix 200 (see FIG. 5) may then be updated. The database
matrix 200 is updated by entering the additional interference
process depicted by the How chart in FIG. 6). Thus, When the
15
telephone (step A2 of FIG. 6). The master controller 380
value p¢(j,k), as calculated above, for the appropriate channel
then identi?es the satellite 12“, 12‘ With the loWest number
of hand-held radio-telephones from the visible satellites and
commands the ?xed radio-telephone 16‘ to steer its direc
j1_4 to Which the neW user terminal 13‘ Was assigned in the
tional antenna 13b‘ so as to aim at that satellite. The
satellites and beams through Which the return link is estab
lished. For example, if FDM channel 190A (i.e., j=1) is
communication link betWeen the ?xed radio-telephone and
gateWay is then established through that satellite. This
deemed the appropriate FDM channel j1_4 to assign the user
terminal 13‘ (in any of steps B2, B4, B7 or B9 of the
further minimiZes the interference in an FDM channel Which
is serving handheld user terminals. In the case of a ?xed
procedure in FIG. 9) the total interference (IO/N0) value for
channel 190A (j=1) in all of the visible satellites 12“, 12‘ in
radio-telephone With a non-steerable antenna, the directional
25
the database are replaced With the calculated total interfer
ence values p(1,k) for the subject channel and satellites 12“,
12‘. When the user terminal 13‘ requests a termination of
services, the additional interference (Ale/N0) for the subject
channel 190A and visible satellites 12“, 12‘, as calculated
above, may then be subtracted from the updated p¢(1,k) value
in the database and the neW updated total interference
density value stored in the database matrix 200. As each neW
user terminal requests and terminates service, this process is
repeated With the interference Within each FDM channel 190
of each beam of each satellite (registered in the database
nature of the antenna may otherWise be utiliZed to allocate
that user terminal to a satellite 12“, 12‘ With a minimum
number of hand-held user terminals. Here, at the time the
?xed user terminal requests service, the master controller
380 of the GOCC 38 is otherWise aWare that the terminal is
a ?xed radio-telephone and also of the orientation of the ?eld
of vieW of the non-steerable directional antenna of the
terminal. The master controller 380 may then proceed to
assign the ?xed radio-telephone to the satellite used by a
minimum of the hand-held user terminals Which is Within
35 the ?eld of vieW of the non-steerable directional antenna of
the ?xed radio-telephone.
matrix 200) being continually updated. This database matrix
In an alternate embodiment, use of the directional capa
200 thus represents a continuous mapping of the overall
interference to thermal noise densities Within each of the
satellites 12 in the constellation of satellites of the commu
nication system 10, and is used to optimally assign neW user
terminals Within the set of available FDM channels 190.
It is also Within the scope of this invention to use the
directional capability of ?xed user terminals 16 With direc
tional antennas to minimiZe the interference in a return link 45
FDM channel 190. The directional antennas may include
antennas that are steerable and-non steerable. The steerable,
or pointable, antennas may be mechanically steerable (e.g.,
by using a gimball) or electronically steerable. The steerable
bility of ?xed user terminals 16 may be accomplished by the
gateWay 18. The type of user terminals already communi
cating on the satellite may be registered by the gateWay 18
and, When the ?xed radio-telephone 16‘ With a steerable
directional antenna request services from the gateWay 18,
the gateWay 18 locates the position of the terminal and
identi?es the terminal as being a ?xed radio-telephone (step
A2 of FIG. 6). The gateWay 18 then identi?es the satellite
12“, 12‘ With the loWest number of hand-held radio
telephones from the visible satellites and commands the
?xed radio-telephone 16‘ to steer, or point, its directional
antenna 13b‘ so as to aim at that satellite. The communica
antennas may also include those capable of producing a
number of ?xed directional beams, and steering may be
tion link betWeen the ?xed radio-telephone and gateWay is
accomplished by beam selection. Referring still to FIG. 4,
user terminal With a non-steerable antenna requests service,
the gateWay 18 is otherWise aWare that the terminal is a ?xed
then established through that satellite. At the time a ?xed
included among the clusters of user terminals 13, 13‘ com
municating through covisible satellites 12“, 12‘ are ?xed
radio-telephones 16‘ With directional antennas 13b‘. Some of
the ?xed radio-telephones 16‘ of the satellite communication
55
system 10 may have steerable directional antennas 13b‘ so
that the antenna may track a satellite 12“, 12‘ along its orbital
path. Other ?xed radio telephones of the communication
system 10 may have substantially non-steerable directional
antennas (not shoWn). In the case of ?xed radio-telephones
radio-telephone and also of the orientation of the ?eld of
vieW of the non-steerable directional antenna of the terminal.
The gateWay 18 may then proceed to assign the ?xed
radio-telephone to the satellite used by a minimum of the
hand-held user terminals Which is Within the ?eld of vieW of
the non-steerable directional antenna of the ?xed radio
telephone.
In both cases above, the system 10 minimiZes interference
With steerable directional antennas 13b‘, the antenna 13b‘
may be pointed at a satellite 12“, 12‘ Which has a minimum
in FDM channels Where hand-held user terminals are
number of hand-held radio-telephones already allocated
thereto. An overall indication as to the number of user 65
telephones is generally more limited (due to limited battery
poWer, desire to extend talk time). The typically high
terminals already allocated to a given satellite 12“, 12‘ may
be obtained from the updated database matrix 200 which
antenna gain characteristic of ?xed radio-telephones as Well
as the ability to operate nominally at a loWer energy per bit
assigned, because the transmit poWer of hand-held radio
US 6,628,921 B1
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18
smaller than an elevation angle of a second one of the
to noise density rate Eb/(NO+IO) than mobile user terminals
(because the ?xed user terminals remain stationary With a
clear line-of-sight to the satellites and operate in an additive
tWo satellites; and
if yes, allocating the third channel to the ?rst user termi
nal.
4. A method as in claim 3, Wherein if the predicted total
interference in the third channel is not less than the prede
termined threshold, or if the elevation angle of the ?rst
satellite is not smaller than the elevation angle of the second
White Gaussian noise propagation channel) makes the ?xed
radio-telephones 16 preferable to assign to FDM channels
containing hand-held user terminals.
The present invention provides for a more optimal loading
of the satellites 12 in the satellite communication system 10
(see FIG. 1), With a concomitant improvement in system
capacity and a reduction in user terminal transmit poWer
needs.
While the invention has been described as utiliZing the
master controller 380 of the GOCC 38 or, in an alternative
satellite, the method further comprises the step of allocating
10
embodiment, as using circuitry in the gateWay 18, it should
be realiZed that the invention is not limited to being achieved
either in the master controller 380 or the gateWay 18. The
15
of channels of the return link to each satellite at a
implemented in a manner that includes both the master
predetermined initial time; and
controller 380 and the gateWay 18.
updating the interference in each channel of the plurality
of channels of the return link to each satellite by adding
Although described in the context of a DS-CDMA com
munication system, it should be realiZed that this invention
also has applicability to other satellite communication sys
the interference of each user terminal allocated to a
corresponding one of the channels and subtracting the
25
applied to other than loW earth orbit (LEO) satellite com
munication systems, such as medium earth orbit (MEO)
satellite communication systems, or geo-synchronous
channel of the plurality of channels of the return link to each
initial time.
7. A method as in claim 1, further comprising the step of
registering the total interference in each channel of the
plurality of channels of the return link to each satellite in a
Thus, While this invention has been particularly shoWn
and described With respect to preferred embodiments
thereof, it Will be understood by those skilled in the art that
changes in form and detail may be made therein Without
departing from the scope and spirit of this invention.
35
?nding a total interference in each channel of a plurality
of channels Which subdivide a predetermined fre
satellite communication system registers other radio fre
quency band of a return link to each satellite from a
quency services located proximate to the ?rst user terminal.
10. A method as in claim 1, Wherein the predicted total
interference is calculated When the ?rst user terminal
plurality of satellites of the communication system;
calculating a predicted total interference from an addition
requests service.
of a ?rst user terminal on each channel of the plurality
channels in the return link to each of at least tWo
45
11. A method for assigning a frequency channel to a user
terminal of a satellite communication system, the user
terminal being illuminated by at least tWo satellites from a
determining if the predicted total interference in a ?rst
channel of the plurality of channels is a minimum value
relative to all predicted total interference values; and
allocating the ?rst channel to the ?rst user terminal if the
predicted total interference in the ?rst channel is the
minimum value.
2. A method as in claim 1, Wherein if the predicted total
plurality of satellites of the communication system, Wherein
the method comprises the steps of:
identifying at least one of a location and a type of the user
terminal When the user terminal requests service;
determining if a ?rst frequency channel from a plurality of
frequency channels for a return link of the user terminal
to each one of the tWo satellites has a minimum total
interference in the ?rst channel is not a minimum, the
method further comprises the step of determining if the
database of the satellite communication system.
8. Amethod as in claim 1, Wherein the step of calculating
a predicted total interference comprises the step of identi
fying a location and type of the ?rst user terminal When the
user terminal request service.
9. A method as in claim 1, Wherein a processor of the
munication system comprising the steps of:
satellites of the plurality of satellites;
interference of each user terminal Which terminates
service on the corresponding channel.
6. Amethod as in claim 5, Wherein the interference in each
satellite is updated at predetermined time periods after the
(GEO) satellite communication systems.
What is claimed is:
1. A method for maximiZing capacity of a satellite com
density across said ?rst and second satellites, from the
combined interference density across said ?rst and second
satellites for all available channels.
5. A method as in claim 1, Wherein the step of ?nding the
total interference comprises the steps of:
calculating an interference in each channel of the plurality
invention may be implemented exclusively in the master
controller 380, or exclusively in the gateWay 18, or may be
tems that utiliZe, by example, Time Division Multiple
Access (TDMA) techniques. This invention may also be
the user terminal a fourth channel, Wherein the fourth
channel exhibits a minimum combined average interference
55
predicted total interference in a second channel is less than
interference density relative to the plurality of fre
quency channels;
a predetermined threshold value; and allocating the second
if yes, assigning the ?rst frequency channel to the user
channel to the ?rst user terminal if the predicted total
interference in the second channel is less than the threshold
value.
3. A method as in claim 2, Wherein if the predicted total
interference in the second channel is not less than the
if no, then determining if a second frequency channel
from the plurality of frequency channels for the return
threshold value, the method further comprises the steps of:
determining if the predicted total interference in a third
channel is less than the threshold value;
if yes, then determining if the ?rst satellite is at an
elevation angle, relative to the user terminal, that is
terminal;
link of the user terminal to each satellite has a total
interference density beloW a predetermined threshold;
if yes, assigning the second frequency channel to the user
terminal;
65
if no, then determining if a third frequency channel from
the plurality of frequency channels has a total interfer
ence density beloW the predetermined threshold for the
US 6,628,921 B1
19
20
return link of the user terminal to a ?rst one of the tWo
minal to the ?rst satellite is above the predetermined
satellites and a total interference density above the
predetermined threshold for the return link of the user
threshold, then assigning a fourth frequency channel
from the plurality of frequency channels for the return
terminal to a second one of the tWo satellites;
if yes, then determining if the ?rst satellite is at a loWer
elevation With respect to the user terminal than the
second satellite;
if yes, then assigning the third channel to the user termi
nal; and
if no, or if the total interference density in the third
frequency channel for the return link of the user ter
5
link of the user terminal; Wherein
the fourth frequency channel exhibits a minimum com
bined average interference density across said ?rst
and second satellites, relative to the combined inter
ference density across said ?rst and second satellites
for all available channels.